The failure of the regular and emergency cooling systems of a nuclear reactor may result in melting of the fuel as a result of the continuous evolution of heat from the decay of fission products therein. Under some conceivable conditions, the melted fuel could penetrate the primary reactor vessel and fall to the floor of the containment building which encloses it. Most existing reactor plants are not designed for such a rare eventuality, and the large mass (in the order of 100 tons) of self-heating heavy fuel (uranium oxide and fission products) could attack the floor of the containment structure, generating gas which could rupture the building by exceeding its survival pressure, or it could melt its way through the foundation. Radioactive material would escape to the biosphere with either type of failure.
Certain analyses have indicated that the probability of a core melt-down is relatively low. However, recent failure experience shows that failure analysis is subject to many uncertainties, and indicates the need for a reliable and relatively invulnerable low cost structure for guarding against the disastrous possibilities which could result from a melt-down. Further, the presence of such an ultimate fail-safe structure might reduce the clamor from anti-nuclear groups which may otherwise reduce the number of nuclear plants which will be built and/or operated.
Various catchments intended to intercept the melted-down reactor fuel mass and prevent rupture of the containment have been proposed heretofore. Two such arrangements are disclosed in U.S. Pat. Nos. 3,607,630 granted Sept. 21, 1971, and 4,036,688, granted July 19, 1977. Other references showing prior proposals of others are cited below in this specification. None of these appear to be practical solutions to the problem, all having one or more of the following deficiencies:
1. They would only decay, instead of preventing the progress of the core material to the outside. PA1 2. They would require some kind of unspecified active devices to remove decay heat, such as pumps, motors, and source of power. PA1 3. They would require complete redesign of the reactor, the containment building, or both, because of their requirements of size, shape, and position. PA1 4. Their cost may be prohibitive. PA1 (1) A blast or explosion isolation conduit extending downward from the main floor of the nuclear reactor plant immediately below the reactor core, and with only a relatively thin sealing layer extending over the upper end of the isolation conduit; PA1 (2) arrangements in the isolation conduit for slowing the descent of the molten material, absorbing the mechanical force or shock of the disruptive events associated with the melt-down, and preventing liquid water accumulation prior to the arrival of the core material, and the possibility of a steam explosion; PA1 (3) A core-catching heat exchanger coupled to the isolation tube to receive the molten material and hold it in a sub-critical geometric configuration; and PA1 (4) Passive heat exchange arrangements for collecting heat both from the core-catching heat exchanger and from the enclosing containment structure, and for discharging the heat into the surroundings, without the release of radiation. PA1 (A) The blast isolation conduit and the core-catching heat exchanger may be formed as a single long vertically extending channel of relatively small cross-section, to facilitate retrofitting to existing reactors. PA1 (B) Cooling tower arrangements located near the reactor containment structure may include passively circulated water cooling arrangements coupled to both the core-catcher heat exchanger and also to a heat exchanger structure extending within the above-ground containment shell. PA1 (C) The isolation conduit and/or the core-catcher heat exchanger may be partially filled with layers of energy absorbing material such as sand, and thin transverse structural supporting sheets, to control the descent of the molten core material. PA1 (D) The isolation conduit and the core-catcher heat exchanger may be enclosed in a massive reinforced concrete shell which is continuous with the containment structure. PA1 (E) The isolation conduit may include an inner wall of steel, covering a layer of refractory material, such as carbon. PA1 (F) The core-catcher heat exchanger may include a steel inner liner and an enclosing water jacket to produce a solid or "frozen" layer of the uranium oxide or other core material against the steel walls as the main mass of the molten material is gradually cooled down. PA1 (G) The core-catcher heat exchanger is sufficiently elongated and thin that the contained core material will not assume a chain reacting or "critical" configuration; and in some cases it may include a bifurcated or branching structure to achieve this result. PA1 (H) Concerning specific dimensions, to facilitate retrofitting, it is contemplated that, as compared with the normal diameter of a reactor containment structure of 100 feet or more (30 or 40 meters or more), the diameter of the isolation conduit and the core-catcher heat exchanger might be in the order of two or three meters (6 or 10 feet), to insure blast or explosion isolation, and to facilitate the digging of a vertical shaft under the reactor structure without seriously undermining it. For fast breeder reactors the core-catcher heat exchanger could narrow down or be bifurcated to a transverse dimension in the order of one foot or about thirty centimeters to avoid the possibility of a "critical" geometry. PA1 1. Collection and guidance of the core debris to the desired location, while preventing attack on the containment building structure and foundations. PA1 2. Control of the maximum temperature and rate of descent of the core debris, while ensuring its final arrival at the desired location. PA1 3. Accommodation to the presence of, or the absence of, larger quantities of water which may be present from previous attempts to prevent melting of the core. PA1 4. Control of the shape and position of the core material at its final location in order to insure a suitable geometry for removal of heat, and to insure that a critical mass cannot be formed. PA1 5. Accommodation to the presence of other residues which may accompany the core material, such as molten steel, structural fragments, etc. PA1 6. Provision of a chilled-wall crucible or core-catching heat-exchanger of such a shape, type, and position that the molten core material is forced to provide a container or solid liner for itself, such self-container consisting of a layer of "frozen" or solid fuel material adjacent to the chilled surface and having sufficient surface area and suitable thickness as to transfer continuously the amount of the decay heat produced in the core material to the chilled wall of the crucible or heat-exchanger. PA1 7. Passive, self-operating arrangements for collecting and conveying heat which is produced by the core material within the building, and of conveying such heat to a suitable point for atmospheric discharge. This can be accomplished by passive circulation of water or air, or by a massive heat conduction system. PA1 8. Protective shielding for insuring that the final chilled-wall crucible and its heat removal system, and the heat removal system for water or steam within the building are not damaged by violent or explosive events during the destruction and melt-down of the reactor vessel and core. PA1 10. Atmospheric discharge arrangements for the passive, self-operating discard of the heat from the two heat-transfer systems, without the discharge of any radioactive materials. PA1 11. Provision is made for removal of the decay heat (heat generated as radioactive material decays) by passive means regardless of the presence or absence of water in the containment and of the degree of interaction of the core material therewith.
A system requiring active devices is clearly unsuitable because it would be unreasonable to assume that the destructive events which have disabled all of the multiple active cooling systems of the reactor have not also damaged the core-catching cooling system or its power supply. A delaying action by itself serves no purpose unless some further system can be brought into play for final retainment. A system which cannot be conveniently retrofitted to existing plants is of little use to an industry which already has on hand over 150 reactors, complete or under construction; and, without greater public confidence, this industry may not ever be able to build any more of these needed power generating reactors.